All automotive vehicles are now equipped with electric lights at their rear ends, including left turn and right turn lights and a tail light which also functions as a stop light. When it is desired to draw a trailer behind a vehicle, the trailer masks the signal lights of the towing vehicle, and so the lights must be duplicated at the rear of the trailer, and electrical connections must be made to the lighting circuits of the car, to operate the trailer lights, as well.
Standard male and female multi-conductor connectors have been developed for this purpose. There has, however, been no standard system agreed on for connecting the conductors of a car or those of a trailer to particular contacts in the standard connectors. Accordingly, the owner of a car having an electrical socket for connection to a first trailer cannot count on being able to connect that socket to the plug of some other trailer.